This invention relates to a document guide and recognition apparatus for machine reading of alpha-numeric characters that appear on the face of a document. It constitutes a manually fed slot reader, where the document is moved longitudinally by the user through a defined path on the equipment in order that its contents might be optically scanned and encoded into a machine readable code. The apparatus constitutes a self-contained remote peripheral which can be coupled to a host processor.
One problem encountered in the design of a compact self-enclosed scanner is the limitations of normal lamp life in the lamps used for illuminating the face of the document across a transverse scan line. Lamp life becomes critical to projected servicing of the equipment, particularly when access to the lamps is difficult or can be accomplished only under factory servicing conditions.
According to this disclosure lamp life is extended by dimming the lamps when they are not in use. Sensors are provided along the document path to sense the presence of incoming documents and automatically increase the brilliance of the lamp illumination for scanning purposes. When no document is present in the apparatus, the voltage drop across the lamps is reduced to a level at which the lamp excitation is slightly beyond the level at which the lamp filament material would return to a crystalline state. By eliminating the crystalline state of the filament, filament damage due to vibration and aging is substantially reduced.
In addition, lamp life is extended by utilizing a constant current source in the power supply for the lamps when they are operational. A constant current flow through the lamps counters the normal tendency of the lamps to be subjected to increased electrical current as they age, which is brought about by the progressive reduction in filament resistance. These electrical systems have substantially extended projected lamp life in the apparatus and have effected reductions in the frequency of service required to maintain proper lamp intensity for scanning purposes.